Drying by induction heating

ABSTRACT

THE DRYING OF INKS OR OTHER COATINGS BY HIGH EFFICIENCY HYSTERESIS-TYPE INDUCTION HEATING OF SELECTED OXIDE PARTICLES THEREIN. THE TERMOHYSTERETIC MATERIALS USED ARE GAMMA FE2O3 AND CRO2 WHICH MATERIALS MAY FORM AT LEAST A PORTION OF THE PIGMENT FOR THE INK. THESE MATERIALS EXHIBIT A RELATIVELY HIGH MAGNETIC SATURATION FLUX DENSITY AND A RELATIVELY HIGH COERCIVE FORCE IN THE PREFERRED LOW FREQUENCY INDUCTION HEATING RANGE OF BELOW 5 MEGACYCLYES PER SECOND AND THUS SUBSTANTIAL HEAT CAN BE GENERATED BY MAGNETIC HYSTERESIS LOSS.

1972 w. c. HELLER. JR HAL 3,705,819

DRYING BY INDUCTION HEATING Filed June 15. 1970- awwx rune.

United States Patent 3,705,819 DRYING BY INDUCTION HEATINQ William C. Heller, Jr., 3521 N. Shepard Ave., Milwaukee, Wis. 53211, and Alfred F. Leatherman, Columbus, and Robert L. Christian, Westerville, Ohio; sa d Leatherman and said Christian assignors to said William C. Heller, Jr., Milwaukee, Wis. Continuation-impart of application Ser. No. 690,883, Dec. 15, 1967. This application June 15, 1970, Ser. No. 46,433

Int. Cl. B41m 7/00 US. Cl. 117-932 7 Claims ABSTRACT OF THE DISCLOSURE The drying of inks or other coatings by high efficiency hysteresis-type induction heating of selected oxide particles therein. The thermohysteretic materials used are gamma Fe O and CrO which materials may form at least a portion of the pigment for the ink. These materials exhibit a relatively high magnetic saturation flux density and a relatively high coercive force in the preferred low frequency induction heating range of below megacyclyes per second and thus substantial heat can be generated by magnetic hysteresis loss.

CROSS REFERENCE TO RELATED APPLICATION The present application is a continuation-in-part application of copending application Ser. No. 690,883, filed Dec. 15, 1967, now abandoned.

BACKGROUND Various types of printing and coating procedures have heretofore been developed and are commonly used for the preparation of printed pages and, in general, for producing coated articles wherever a continuous, patterned, or decorative coating is desired. -In the production of such coatings, numerous types of machines and processes are employed, such as letterpress equipment, offset printing, silkscreen, gravure, flexographic, and doctoring techniques. Of particular interest are those applications of high quality where good uniform copy is desired on highquality paper, wherein many copies must be printed and high press speeds are therefore employed. In these and many other printing and coating operations, the ink or coating is applied wet and must then be dried or brought to an apparently dry state quickly in order to permit handling without smearing, blotting, or blocking. Likewise, quick drying is important in certain color printing work to permit one color to become substantially dry or set before application of another color, and generally to prevent the transfer of wet portions of the copy to other sheets commonly referred to as offset.

Various means for accelerating drying to prevent offset or the like are employed commercially or have been heretofore proposed. Some methods use gas flames impinging onto the printed or coated webs, sheets or objects to accelerate drying or oxidation. Other methods use corona discharges, ozone gas, electric arcs, dielectric heating, resistance heating, electrostatic attraction of vapor, and radiant heating. Generally, these previously proposed methods have certain disadvantages as hereinafter noted.

The present invention provides a novel induction heating method in which the heat for drying the deposit is generated directly and exclusively in the deposit itself. Therefore, any tendency for a substrate material to become heated is restricted to only those areas at which the deposit is present, thus providing improved opportunity Patented Dec. 12, 1972 for proper control, avoiding over-drying, and eliminating shrinking or puckering of some plastic films. Also, in certain compositions, the presence of heat at the deposit can augment or provide adhesion of the deposit to polyolefin films without the usual need to pretreat the film by corona or flame methods.

The effective heating action of the present invention oflfers less web travel in drying that is required by conventional drying ovens, making for easier starting, less floor space requirement, and less waste generation or scrap upon shut-down. Since the heat is generated directly in the deposit, a drying oven is not required, and therefore no need exists to preheat or maintain heat in an oven. Additionally, since no oven exists, high temperature environment in the press room or drying room is no longer a problem.

Since the usual oven or flame source in conventional drying must operate at temperatures somewhat above those that can damage a non-moving web, scrap losses can occur from overheating if the conventional processing line must be stopped suddenly. Electrical control can be used in the present invention to stop the drying action immediately upon stopping of processing. When processing is then started again, all work in process can generally be salvaged with the present process.

With the conventional printing methods, there is often a tendency for an outer crust of the deposit to dry first and prevent the liberation of vapors from the ink inside the crust when inks are used which require solvent re moval. However, with the present invention, uniform generation of the heat throughout the deposit produces uniform drying and reduces this tendency.

In conventional manufacture of laminated webs, such as polyethylene film on foil, or Mylar laminated film to another plastic film, it is often desired to print on one of the members as a substrate and then laminate the other film over the printed deposit. Often the ink formulation required for fast drying and adhesion of the printed deposit to the substrate will be of a type that will tend to dissolve as a result of the adhesive used for the laminating. This effect can cause bleeding of the printed information. The opportunity exists with the present invention to use radically different ink formulations for printing the substrate, since the heat generated in the new inks will promote rapid setting of inks that might otherwise set too slowly to be used at all. Therefore, an ink composition can be chosen that will not dissolve and/ or bleed during laminating.

With the present invention, drying of deposits of irregular thickness, such as obtained in fiexography, occurs more uniformly since the thicker deposits automatically tend to generate heat more rapidly than thin ones and surface skin-drying is a lesser problem. Drying of oily type ink deposits on plastic is also assisted since higher localized temperatures are available. Any tendency for plastic disorientation during drying is furthermore eliminated or at least localized in comparison with conventional methods. In the drying of paperboard where moisture level is important, moisture can be added to the paperboard at the same time that drying of printed deposits is taking place.

In many conventional printing methods, sufiicient time does not exist to permit the inks to dry and it is necessary to apply a powder type material to the sheets to separate them and reduce the tendency for blocking and smearing. This powder type material tends to disperse and spread, often causing problems in plant cleanliness, machinery maintenance, personnel cleanliness, air pollution, and residues are included with the final printed material, thereby indicating an inferior product or manufacturing procedure to a customer. The present invention provides new inks and ink drying procedures that produce heat energy within the inks themselves so as to greatly accelerate the completion of curing, drying, or setting, with the elimination of the need for the powdery material referred to.

The improved drying techniques discussed are found to be of particular value for the drying of heavy deposits such as are obtained in silk screen application as usually required for printing on rigid plastics. Such coatings to which the present method is applicable may be up to about one-eighth of an inch in thickness. Accordingly, the term thin or thin deposit is intended herein to include such coatings, and the term finely divided is intended to include all particles that are not so large as to quirements, low heating rates, need for complicated and expensive high frequency equipment, and sensitivity to paper thickness, type, and moisture content.

The prior art has further suggested the use of antiferromagnetic materials, such as ferrites, ferrites being Fe O plus the addition of a divalent metallic ion, such as cobalt, magnesium, zinc, cadmium, manganese, iron, etc. While such ferrite particles will work, they suffer several disadvantages. The major disadvantage is the fact that extremely high frequency must be used to generate heat at a practical rate. When such extremely high frequencies are used, the coils necessary for generating the magnetic field become very small physically because of the necessity of establishing resonance with the power supply. Such small coils make it difiicult to dry printed materials which are even modest in size and completely preclude drying large size materials such as newsprint.

The use of antiferromagnetic ferrite materials also requires the use of a fairly intense magnetic field to achieve satisfactory results. This requires radio interference shielding for the drying chamber. Additionally, the size of the ferrite particles is quite critical to the results.

SUMMARY The present invention, therefore, provide an improved method of ink drying by means of thermohystereticparticles having the advantage of providing rapid and efficient v drying of inked or other coatings on large sheets.

The method is characterized by the use of gamma Pe o or CrO particles in the ink rather than the antiferromagnetic, ferrite particles, of the prior art. The aforesaid particles, which are of the nonferrite type, have both a high saturation flux density and high magnetic coercivity which provide desirable hysteresis heating properties to the, particles.

The particles, of which may form at least a portion of the ink pigment, may be used in a wide variety of sizes. 'For viscous inks, the particles are preferably smaller than about 20 microns in the greatest dimension. However, for particularly thin coatings, they should be smaller than about 3 microns in at least one dimension.

The magnetic field frequency for the induction heating is preferably about 0.5 to 30 megacycles per second. In practice, it is found, as known in the art, that a coil for use at 30 megacycles per second cannot be physically large. Reasonably large coils have been found to exist for use with frequencies up to about 5 megacycles per second. The preferred frequency range for the actual ink drying application with wide printed webs is thereforeabout 0.5 to 5 megacycles per second.

The ink or other coating carrier for the particles may comprise a material, such as a dissolved or dispersed resinous binder, which solidifies by evaporation so that the heating accelerates the removal of a volatile component in the composite. It may comprise a mixture of materials, such as plastisols or organosols that convert toa homogeneous fluid upon heating and solidify upon subsequent cooling. The ink may comprise a thermosetting material such as heat-curable monomer or resin or a reactive system that cures at elevated temperatures such as a blocked isocyanate-polyol or an epoxy-anhydride system.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a transverse sectional view of a typical deposit or coating in an arbitrary pattern on a substrate in the form of a sheet; and

FIG. 2 is a somewhat schematic perspective view of a typical sheet provided with a printed deposit and with an induction coil encircling the printed sheet for drying the deposit according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a cross section of a substrate 10 the form of a paperboard web is shown. A web impression 11 of suitable printed or coating materials has been deposited upon the web by a suitable technique such as letterpress, gravure, offset, silkscreen, electro static method, doctor blade, or other methods. The deposit 11 contains non ferrite, oxide particles of gamma Fe Og or CrO as a heat generating (thermohysteretic) constituent. These particles feature both a high saturation flux density and a high magnetic coercive force. The Fe O must be the hematite, gamma type of ferric oxide rather than the colcothar alfa type of ferric oxide.

The thermohysteretic particles employed in practicing this invention are electrically poor conductors and are typically less than 20 microns in their longest dimension, but for particularly thin coatings, they should be less than 3 microns in at least one dimension. Their small size make it practical to use such particles in ink and other coatings.

The particles can serve both as the heat-generating medium and the pigment for inks. For example, gamma Fe Og is brown and CrO is black. Conventional pigments in inks rangein size from a recommended maximum size of 18 microns for news ink, to a maximum of 8 microns for good quality letterpress ink. [See Apps, Ink Technology 'for Printers and Students, volume 1, page 3 (1964)]. Since the thermohysteretic particles tend to have a higher density than other pigments, a slightly smaller size than is conventional is preferred.

In FIG. 2, the web 10 is shown moving through an induction heating coil 15, which is connected to a source of high frequency energy 16. As the web progresses through the energized coil, heat is generated in deposits 11 (shown herein as comprising the printed letters, A, B, C for purposes of illustration) by action of the highfrequency field 17, flux lines of which are shown schematically by the dotted lines. The amount of heat generated is proportional to the rate of travel of web 10 (i.e. exposure time) and the strength of field 17. As noted supra, the frequency of magnetic field 17 is preferably 0.5 to 5 megacycles per second. The use of magnetic fields in this frequency range facilitates obtaining the desired field strength.

The magnetic field quickly heats the thermohysteretic particlesby virtue of the hysteresis type of induction heating. The hot particles, in turn, quickly heat the web deposits and cause it to dry or harden.

In contrast to the more common eddy-current type of induction heating, the hysteresis type of induction heating functions wellto heat very small particles as .well as large ones. Eddy-current heating is limited to particles larger than a certain size dictated by the material and the frequency of the applied magnetic field, as known in the art. The particle size for efficient eddy-current heating has been found to be too large for practical use of the particles in inks without excessive settling-out of the particles. When particles sufiiciently small to avoid settling are used, an impracticall'y high magnetic field frequency is required to obtain eddy-current heating. The present invention overcomes the problem of induction heat generation in small particles by using the proper particles, frequencies, and ink components in the manner of this invention.

The practical utility of the present invention has been demonstrated in the following specific example. A solvent type ink was prepared by mixing 100 parts by Weight of gamma Fe O particles about 1 micron and smaller in at least one dimension, with 100 parts of a rosin maleic resin (e.g. Amberol 801 resin by Rohm and Haas). The ink was then made into a liquid by adding enough solvent to bring viscosity to a useful level. The solvent used can be a high-boiling hydrocarbon solvent (e.g., Solvent and Chemicals No. 450 by Amsco). This ink was then deposited by means of a doctor blade onto a piece of paper in deposit thicknesses varying from translucent to fully opaque. The paper specimen was then passed through the opening of a hairpin-type induction heating coil connected to a commercial Ill-kilowatt induction heater operating at a frequency of about 4 megacycles per second. The singleloop coil carried about 250 amperes of current. The ink was within the effective region of the magnetic field for about 0.1 second. A small amount of fumes was liberated as drying took place. Immediately after the processing, the deposit was dry to the touch.

In other embodiments of the present invention, the thermohysteretic particles serve to generate heat which causes a thermoplastic, thermosetting, or oxidizing action to take place to dry the coating. Externally, these embodiments do not differ in appearance from the schematic arrangements shown in the drawing, but the deposit itself does differ. Insofar as the deposit is concerned, several types of thermoplastic, oxidizing, and thermosetting coatings or systems are already known to the art, into which thermohysteretic particles can be added in accordance with this invention.

An example of one type of thermoplastic ink system in which the heat-generating procedure of the present invention can be used is a mixture containing one-half thermohysteretic particles and one-half plastisol by weight. Plastisol materials incorporating such components as solid polyamides or vinyl chloride polymers or copolymers, for example, are characterized by a fluid, two-phase dispersion of resin in plasticizers near room temperature. These materials convert to a single-phase plasticized resin when heated to about 350 F. Upon, cooling, the plastisol solidifies and thus becomes dry.

An example of a thermosetting ink is a liquid mixture of a liquid reactive epoxy resin (e.g. Epon 815 by Shell Chemical Corporation) and methyl nadic anhydride prepared in the ratio of 4 to 1 by volume. Three parts thermohysteretic particles are added to ten parts by weight of liquid. When the ink temperatures reach about 350 F., epoxidation takes place, causing the deposit to become firm and dry. Since no cooling or vapor evolution is necessary in the embodiment to complete the reaction, it is particularly useful for thick deposits.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

We claim:

1. A drying method comprising the steps of:

providing a carrier dryable by heating;

distributing in the carrier finely divided oxide particles selected from a class consisting of gamma E2 0 and CrO said particles having high saturation flux density and magnetic coercivity properties providing large area hysteresis loop characteristics to the particles; depositing the carrier on a substrate; and

subjecting the particles to an alternating magnetic field of 10 megacycles per second or less for generating heat from the hysteresis characteristics of the particles to dry the carrier on the substrate.

2. The drying method of claim 1 wherein the step of subjecting the particles to an alternating magnetic field is further defined as subjecting the particles to an alternating magnetic field of between 0.5 and 5 megacycles per sec- 0nd.

3. The drying method of claim 1 wherein the particle distribution step is further defined as distributing in the carrier, particles smaller than about 20 microns in the greatest dimension.

4. The drying method of claim 1 wherein the particle distributing step is further defined as providing the particles as a pigment in the carrier.

5. The drying method of claim 1 wherein the carrier providing step is further defined as providing a carrier which dries by evaporation responsive to the heat of the particles.

6. The drying method of claim 1 wherein the carrier providing step is further defined as providing a carrier of thermosetting material.

7. The drying method of claim 1 wherein the carrier providing step is further defined as providing a carrier comprised of a mixture of materials that convert to a homogeneous fluid upon heating and solidify upon subsequent cooling.

References Cited UNITED STATES PATENTS 3,391,846 7/1968 White 1l793.2

RALPH S. KENDALL, Primary Examiner I. H. NEWSOME, Assistant Examiner US. Cl. X.R. 1l7-38; 34l; 162-492; 219-10.61 

